Field curvature reduction for optical system

ABSTRACT

An optical head ( 1 ) for scanning an optical record carrier ( 2 ) is provided with a compensator ( 16 ) for compensating the field curvature aberration of the objective system ( 18 ) caused by operation of the optical head at a different field angles. A surface of the compensator ( 16 ) comprises a phase structure in the form of annular areas ( 52, 53, 54, 55, 56 ), the areas forming a non-periodic pattern of optical paths of different lengths. The optical paths change as a function of field angle and form a field angle-dependent wavefront deviation that compensates field curvature of the optical head.

The invention relates to an optical head for scanning an optical recordcarrier having an information layer, the head comprising a radiationsource for generating a radiation beam, an optical system for convergingthe radiation beam to a focus on the information layer, the opticalsystem imparting a field angle-dependent first wavefront deviation tothe radiation beam, and a compensator arranged in the radiation beam forcompensating the first wavefront deviation. The invention also relatesto an optical device comprising an optical system and a compensator, theoptical system imparting a field angle-dependent first wavefrontdeviation to a radiation beam passing through the optical device, thecompensator being arranged for compensating the first wavefrontdeviation.

A multi-track optical head scans several tracks on the information layerof an optical record carrier simultaneously. This is achieved byfocussing a number of beams with different field angles to a same numberof spots on the information layer by means of an optical system, whereeach spot scans a track. When the optical system exhibits fieldcurvature wavefront aberration, not all spots will be simultaneously infocus on the information layer, resulting in a deterioration of the readout signals of the tracks for which the spot is not correctly in focus.When the optical system consists of a single lens, the field curvatureis to lowest order proportional to $\begin{matrix}{\left( {c_{1} - c_{2}} \right)\left( {\frac{1}{n} - 1} \right)} & (1)\end{matrix}$with c₁ the curvature of the first surface of the lens, c₂ the curvatureof the second surface of the lens and n the refractive index of thematerial of the lens. In order to have no field curvature, the radii ofthe lens surfaces must be the same, which is in general not practical.This means that resort has to be made to other methods to reduce thefield curvature of the objective lens.

Not only for optical recording but also for other areas of opticaldesign such as photography lenses, motion picture projection objectives,lenses for microlithography, field curvature is a dominating aberrationwhich must be corrected for good performance of the lens.

A method to compensate field curvature of a lens is to add an additionalnegative lens, called “field flattener” (see for instance W. J. Smith,Modern optical engineering, (McGraw-Hill, New York) chapter 13). Thefield flattener has to be positioned near the focal plane of the lens inorder to keep the power contribution of the field flattener to thesystem low. Although it results in a significant reduced fieldcurvature, a drawback of positioning the element close to the focalplane is that it becomes sensitive to dust and other contamination. Foroptical recording such a solution is not practical because of thethickness of the cover layer of the disk. As a result, the fieldflattener is at least at a distance equal to the thickness of the coverlayer of the disk away from the focal plane, which significantly reducesthe action of the field flattener.

Another method to reduce field curvature has been proposed in an articlepublished in Applied Optics vol. 32, (1993) pp. 60-66, where a staircaselens is used to correct field curvature. The staircase lens (or notchedlens) can be considered as a combination of a refractive substrate and ablazed kinoform with zero combined power. Although such a blazedkinoform can be designed to yield 100% efficiency in one particulardiffraction order, actual gratings never attain such high efficiency.This reduction of the efficiency of the light path of the optical headis disadvantageous in optical recorders, which require a large amount ofradiation energy for writing information in the record carrier orreading the information from the record carrier at high speeds. Anotherdrawback of these periodic grating structures is that they contain ingeneral a large number of zones, making them difficult to manufacture.

It is an object of the invention to provide an optical head in which theeffect of field curvature is reduced without the above mentioneddisadvantages.

This object is achieved if, according to the invention, the compensatorincludes a phase structure of a material having field angle-dependentproperties, the phase structure having the form of annular areas forminga non-periodic pattern of optical paths of different, fieldangle-dependent lengths, the optical paths forming a second wavefrontdeviation compensating the field angle-dependent first wavefrontdeviation. If the material of the phase structure and the optical pathsof the annular areas are properly chosen, the phase structure of theannular areas introduce a wavefront deviation in the radiation beamhaving the correct shape and field angle dependence to compensate thewavefront deviation of the optical system. The compensation may be totalor partial. The phase structure does not impose restrictions on theelements of the optical system, thereby leaving a great freedom ofdesign. Another aspect related to this freedom is that the annular areasforming the non-periodic pattern can be made relatively wide, whichimproves the manufacturability significantly at the expense of lessperfect but still sufficient compensation of the wavefront deviation.The manufacturability will be further improved if the optical paths ofneighbouring areas differ by more than one wavelength. A furtheradvantage of the invention is that the compensator can be designed tohave no optical power. As a result, the compensator can be placedanywhere in the lens system, contrary to the conventional fieldflattener, which must be placed close to the focal plane.

It should be noted, that the phase structure according to the inventionhas a non-periodic pattern, and, therefore, does not form diffractionorders. As a consequence, the non-periodic phase structure does not havethe inherent losses of unused diffraction orders of a grating. Thecompensator is therefore very suitable for use in an optical head thatrequires a change in wavefront in dependence on the field angle, becausethe phase structure can introduce the required field angle-dependentwavefront changes without appreciable loss of radiation energy.

In a preferred embodiment the optical head includes an objective systemimparting field curvature aberration as the first wavefront deviation tothe radiation beam. When the wavefront incident on the compensator has aradius of curvature different from the radius of curvature of thesurface of the compensator, the compensator can be designed tocompensate not only field curvature but also coma wavefront aberration.

Preferably, the non-periodic phase structure compensates at least 50% ofthe root mean square (rms) field curvature wavefront error at a certainfield angle with respect to the direction of an optical axis of theoptical system and caused by the objective. More preferably, thecompensation is at least 70%.

Preferably, the rms wavefront error caused by the field curvaturegenerated by the objective lens at a maximum required field angle withrespect to the axial direction and compensated by the non-periodic phasestructure is less than 40 mλ. More preferably, the wavefront error isless than 20 mλ.

Another aspect of the invention relates to a scanning device forscanning an optical record carrier having an information layer, thedevice including an optical head according to the invention.

A further aspect of the invention relates to an optical device includingan optical system and a compensator, the optical system imparting afield angle-dependent first wavefront deviation to a radiation beampassing through the optical system, the compensator being arranged inthe path of the radiation beam for compensating the first wavefrontdeviation, in which the compensator comprises a phase structure havingfield angle-dependent properties, the phase structure having the form ofannular areas forming a non-periodic pattern of optical paths ofdifferent, field angle-dependent lengths, the optical paths forming asecond wavefront deviation compensating the field angle-dependent firstwavefront deviation. The optical system is preferably an objectivesystem. The optical system may be a refractive and/or a diffractiveand/or a reflective system.

In a preferred embodiment of the optical device the differences betweenthe optical paths at zero field angle are multiples of the firstwavelength. In that case the phase structure will not affect theradiation beam at zero field angle, whereas it will introduce awavefront deviation in the radiation beam at non-zero field angles.

Other optical functions can be integrated in the optical system byarranging a grating on one of the surfaces of an element in the opticalsystem. The grating can be used to make the optical system achromatic orto make a DVD objective compatible for scanning record carriers of theCD type.

Other optical functions can be integrated in the compensator byarranging a grating on one of the surfaces of the optical element. Thegrating can be used to make the lens system achromatic or to make a DVDobjective compatible for scanning record carriers of the CD type.

In a special embodiment of the optical set, the wavefront deviation isfield curvature.

The objects, advantages and features of the invention will be apparentfrom the following more particular description of preferred embodimentsof the invention, as illustrated in the accompanying drawings, in which

FIG. 1 shows a scanning device according to the invention;

FIG. 2 shows a cross-section of the compensator;

FIG. 3 shows the wavefront aberration of the objective lens at anelevated field angle; and

FIG. 4 shows the wavefront aberration of the combination of theobjective lens and the compensator at the elevated field angle.

FIG. 1 shows a device 1 for scanning an optical record carrier 2. Therecord carrier comprises a transparent layer 3, on one side of which aninformation layer 4 is arranged. The side of the information layerfacing away from the transparent layer is protected from environmentalinfluences by a protection layer 5. The side of the transparent layerfacing the device is called the entrance face 6. The transparent layer 3acts as a substrate for the record carrier by providing mechanicalsupport for the information layer. Alternatively, the transparent layermay have the sole function of protecting the information layer, whilethe mechanical support is provided by a layer on the other side of theinformation layer, for instance by the protection layer 5 or by afurther information layer and a transparent layer connected to theinformation layer 4. Information may be stored in the information layer4 of the record carrier in the form of optically detectable marksarranged in substantially parallel, concentric or spiral tracks, notindicated in the Figure. The marks may be in any optically readableform, e.g. in the form of pits, or areas with a reflection coefficientor a direction of magnetization different from their surroundings, or acombination of these forms.

The scanning device 1 is of the so-called multi-track type and includesa radiation source 11 that can emit a radiation beam 9. The radiationsource may be a semiconductor laser. A grating 10 diffracts the beamfrom the radiation source in seven different diffraction orders: −3, −2,−1, 0, 1, 2, 3, respectively, making different angles with the opticalaxis 8. In general, a radiation beam in the following includes the sevendiffraction subbeams. A beam splitter 13 reflects the divergingradiation beam 12, towards a collimator lens 14, which converts thediverging beam 12 into a collimated beam 15. The collimated beam 15 isincident on a transparent compensator 16, which modifies the wavefrontof the collimated beam in dependence on the field angle of each of theseven subbeams in the scanning device. The beam 17 coming from thecompensator 16 is incident on an objective system 18. The objectivesystem may comprise one or more lenses and/or a grating. The objectivesystem 18 has an optical axis 19. The objective system 18 changes thebeam 17 to a converging beam 20, incident on the entrance face 6 of therecord carrier 2. The objective system has a spherical aberrationcorrection adapted for passage of the radiation beam through thethickness of the transparent layer 3. The converging beam 20 forms sevendifferent spots 21 on the information layer 4, each focussing ondifferent neighbouring or close spiral tracks. Radiation reflected bythe information layer 4 forms a diverging beam 22, transformed into asubstantially collimated beam 23 by the objective system 18 andsubsequently into a converging beam 24 by the collimator lens 14. Thebeam splitter 13 separates the forward and reflected beams bytransmitting at least part of the converging beam 24 towards a detectionsystem 25. The detection system captures the radiation of the seven subbeams separately and converts it into electrical output signals 26.Elements 10, 11, 13, 14, 16, 18 and 25 form an optical head in thescanning device. Elements 16 and 18 form a lens system having a reducedfield curvature.

A signal processor 27 converts these output signals to various othersignals. One set of the signals is a set of information signals 28, thevalues of which represents information read by each of the sevensubbeams from the information layer 4. The information signals areprocessed by an information processing unit for error correction 29.Other signals from the signal processor 27 are the focus error signaland radial error signal 30 which are retrieved from the central orzero-order subbeam. The focus error signal represents the axialdifference in height between the spot 21 and the information layer 4.The radial error signal represents the distance in the plane of theinformation layer 4 between the spot 21 and the centre of a track in theinformation layer to be followed by the spot. The focus error signal andthe radial error signal are fed into a servo circuit 31, which convertsthese signals to servo control signals 32 for controlling a focusactuator and a radial actuator respectively. The actuators are not shownin the Figure. The focus actuator controls the position of the objectivesystem 18 in the focus direction 33, thereby controlling the actualposition of the spot 21 such that the spot of the central subbeamcoincides substantially with the plane of the information layer 4. Theradial actuator controls the position of the objective lens 18 in aradial direction 34, thereby controlling the radial position of the spot21 such that the spot of the central sub beam coincides substantiallywith the central line of track to be followed in the information layer4. The tracks in the Figure run in a direction perpendicular to theplane of the Figure. Since the central subbeam is in focus with theinformation layer, the other sub beams may be out of focus when the lenssystem exhibits field curvature, significantly deteriorating the signalquality obtained from these subbeams.

The device of FIG. 1 may be adapted to scan also a second type of recordcarrier having a thicker transparent layer than the record carrier 2.The device may use the radiation beam 12 or a radiation beam having adifferent wavelength for scanning the record carrier of the second type.The NA of this radiation beam may be adapted to the type of recordcarrier. The spherical aberration compensation of the objective systemmust be adapted accordingly.

The objective system 18 shown in the embodiment of FIG. 1 is a singlelens having an NA of 0.6 for operation at a wavelength of 650 nm. Theentrance pupil diameter is 3.3 mm. The lens has one aspherical and oneplanar surface. The aspherical surface is made by applying a thinacrylic layer on a surface of a glass lens body. The lens has athickness on the optical axis of 1.922 mm. The lens body is made ofSFL56 Schott glass with refractive index n=1.7767. The convex surface ofthe lens body which is directed towards the collimator lens has radiusof 2.32 mm. The acrylic layer has refractive index of n=1.5646. Thethickness of this layer on the optical axis is 22 μm. The rotationalsymmetric shape of the aspherical surface can be described by theequationz(r)=B ₂ r ² +B ₄ r ⁴ +B ₆ r ⁶+ . . .with z being the position of the surface in the direction of the opticalaxis in millimetres, r the radial distance to the optical axis inmillimetres, and B_(k) the coefficient of the k-th power of r. Thevalues of B₂ to B₁₀ for the surface of the objective lens facing theradiation source are 0.24137393, 0.0046535966, −0.00014987079,−4.0957635 10⁻⁵, −8.3283927 10⁻⁶, respectively. The distance between theobjective lens and the disk is 1.290 mm. The cover layer of the disk is0.6 mm thick and is made of polycarbonate with refractive indexn=1.5803. The subbeam corresponding to the third order diffraction beamof the seven-spots grating has a field angle of 0.73 degrees on theobjective lens. Due to the field curvature of the objective lens thissubbeam will focus 0.42 μm before the information layer. Since thisdefocus aberration of the outer beam can not be compensated for by theactuator without affecting the central beam, the field curvatureaberration will reduce the quality of the focal spot 21 of the otherthen central subbeams. The compensator 16 is adapted to compensate thefield angle-dependent aberration of the objective lens.

FIG. 2 shows a schematic cross-section of the compensator 16. Thecompensator comprises a transparent plate 50, one surface of which is aphase structure, which is rotationally symmetric around the optical axis19. The phase structure has a central area 51 and five concentricannular areas 52, 53, 54, 55 and 56. The annular areas 52, 53, 54, 55and 56 are rings with a height of h₁, h₂, h₃, h₄ and h₅ above the heightof the central area 51. The height of the areas is exaggerated withrespect to the thickness and radial extent of the plate 50. The ringsare made of a material having a refractive n. The plate 50 may also bemade of the same material as the rings.

The heights h_(j) are each equal to m_(j)h, with m_(j) an integer and hequal to $\begin{matrix}{h = \frac{\lambda}{n - 1}} & (2)\end{matrix}$where λ is the wavelength and n is the refractive index of the materialof the rings at the wavelength. In this particular example thecompensator is made of the material COC with refractive index n=5312. Asa result, the height h is equal to 1.224 μm. Since each of the annularareas with height h_(j) introduces a phase change of (m_(j) 2π) radiansin the radiation beam at the zero field angle, the phase structure doesnot change the wavefront of the radiation beam at this angle. When theincident beam enters the structure at a field angle θ, the height h_(j)no longer introduces a phase change of (m_(j) 2π). The difference phaseΔΦ_(j)□ introduced by ring j into the radiation beam is equal to$\begin{matrix}{{\Delta\Phi}_{J} = {2\pi\quad{m_{j}\left\lbrack {{\frac{1}{n - 1}\left( {{n\sqrt{1 - \frac{\sin^{2}\theta}{n^{2}}}} - {\cos\quad\theta}} \right)} - 1} \right\rbrack}}} & (3)\end{matrix}$For small field angles θ this equation can be simplified to$\begin{matrix}{{\Delta\Phi}_{J} = {\frac{\pi\quad m_{j}}{n}\theta^{2}}} & (4)\end{matrix}$

with the field angle □θ expressed in radians. Consequently, when thebeam enters the compensator with a non-zero field angle θ, □ thecompensator gives rise to a stepped wavefront deviation. By properdesign of the zone widths and heights the non-periodic phase structurecan compensate the field curvature of the objective, which isproportional to θ². If the beam enters the optical head at an fieldangle 0.73 degrees (12.74 mrad), the objective lens introduces 37.5 mλRMS field angle-induced defocus due to the field curvature aberration.FIG. 3 shows a cross-section of the defocus contribution to thewavefront W as a function of the radius r of the radiation beam. Thephase change ΔΦ_(j) introduced by ring j of height m_(j)h of thenon-periodic phase structure at 12.74 mrad field angle is now 0.000333m_(j) radians. The values of the integers m_(j) for each of the rings inthe phase structure must be chosen such that the phase structure willintroduce a wavefront deviation that approximates the defocuscontribution to the wavefront due to the field curvature as shown inFIG. 3 but with opposite sign. Table I shows the results of theoptimisation by the radii of the four annular areas shown in FIG. 2, theheight of each area and the relative phase of the radiation beam afterpassage through each area for 12.74 mrad field angle. TABLE I begin areaend area height m_(j)*h ΔΦ_(j)(θ = 12.74 mrad) (mm) (mm) (μm) m_(j)(radians) 0.0 0.2 0 0 0 0.2 0.5 115.06 94 0.0312 0.5 0.8 358.63 2930.0976 0.8 1.1 779.69 637 0.2121 1.1 1.4 1527.55 1248 0.4155 1.4 1.652476.15 2023 0.6737

FIG. 4 shows the defocus wavefront error at θ=12.74 mrad when both theobjective lens and the compensator are arranged in the radiation beam.The wavefront error is now 9.3 mλ. Consequently, the compensator reducesthe field curvature wavefront aberration of the objective lens caused bythe change in field angle from a defocus wavefront error of 37.5 mλ to9.3 mλ, hence a reduction by a factor of four. Although only thereduction for one value of the field angle has been shown, the reductionfactor of approximately four will also hold for a whole range of fieldangles around the zero field angle used in this embodiment, because boththe field curvature wavefront deviation introduced by the objective lensand by the compensator are quadratically proportional to the fieldangle.

Table I shows that the step height distribution is clearly non-periodic.Furthermore, by letting the difference between subsequent values ofm_(j) be larger than one, the annular areas can be made wide and as aresult the number of annular areas can be small (in this embodimentsix), which makes the structure easier to manufacture. As a result ofthe limited number of annular areas the compensation of the field angleeffect is not perfect, as can be seen in FIG. 4, which shows thewavefront aberration remaining after compensation. Increasing the numberof annular areas results in a lower rest wavefront aberration but alsoin a more difficult to make structure. An advantage of making thestructure non-periodic is that the designer can balance betweencomplexity of the structure versus the wavefront aberration remaining.

Furthermore, table I shows that the absolute value of the step heightsare monotonously increasing as a function of the central radius of eachzone. In the case the field curvature has opposite sign, hence the outersubbeams focus beyond the information layer, the values of m_(j) becomenegative, but the absolute values of m_(j) still are monotonouslyincreasing as a function of the central radius of each zone. Anothercharacteristic of the absolute value of the step heights is that theyincrease more than linearly as a function of the central radius of eachzone. An improved operation of the phase structure is obtained if thewavefront of the incident central beam at the surface on which thenon-periodic phase structure has a shape which is substantially the sameas that of the surface. More specifically, if the wavefront issubstantially spherical, the surface is preferably also substantiallyspherical; in other words, the radii of curvature of the wavefront andthe surface should be substantially the same, preferably the differenceis less than 20%. For example, when the phase structure is arranged on aflat surface, the incident beam should be substantially flat. When bothradii are not substantially the same, the non-periodic phase structurewill, apart from field curvature, also give rise to a comatic wavefrontaberration as disclosed in European patent application 00304997.2(PHNL000659 EP-P). Since the amount of coma introduced can be controlledby the difference in radii of curvature of the wavefront and thesurface, the areas and heights of the non-periodic phase structure canbe designed such, that both field curvature and coma introduced by anembodiment of the objective system 18 can be reduced by the compensator16 for field angles different from zero.

Although the number of zones of the non-periodic phase structure isequal to six in the described embodiment, it may be any number.Preferably, the number of zones is larger than two in order to havesufficient compensation of the wavefront aberration. Preferably, thenumber is less than ten for reasons of manufacturability.

The compensator 16 and the objective system 18, shown in FIG. 1 asseparate elements, may be integrated by arranging the phase structure 51to 56 on a lens surface of the objective system. Preferably, the phasestructure is arranged on an aspheric lens surface. The design of thecompensator may be modified to compensate also any field curvatureintroduced by the collimator lens 14.

It is to be appreciated that numerous variations and modifications maybe employed in relation to the embodiments described above, withoutdeparting from the scope of the invention which is defined in theappended claims. In the embodiment described a plurality of radiationbeams at different field angles pass through the objective systemsimultaneously. The compensation may also be used in combination withobjective systems in which a single beam passes through the objectivesystem. The objective system is shown as a plano-convex lens; howeverother lens element types such as a convex-convex, convex-concave orconcave-concave lenses may also be used. Whilst the objective system inthe described embodiment is a single lens, it may be a compound lenscontaining two or more lens elements, either, or both, of which mayinclude part of the non-periodic phase structure of the invention. Theobjective lens may for example comprise a refractive objective lenselement and/or a diffractive lens element. The application of thenon-periodic phase structure according to the invention is not limitedto the field of optical recording. The phase structure may be used inany field of optics, e.g. photography, motion picture projectionobjectives and lenses for microlithography. In these areas fieldcurvature is a dominating aberration which must be corrected for goodperformance of the lens.

Whilst in the above described embodiment a scanning device for scanningrecord carriers of the so-called DVD format is described, it is to beappreciated that the scanning device can be alternatively oradditionally used for any other types of optical record carries to bescanned. It is also to be appreciated that radiation of otherwavelengths than 650 nm suitable for scanning optical record carriersmay be used.

1. An optical head for scanning an optical record carrier having aninformation layer, the head comprising a radiation source for generatinga radiation beam, an optical system for converging the radiation beam toa focus on the information layer, the optical system imparting a fieldangle-dependent first wavefront deviation to the radiation beam, and acompensator arranged in the radiation beam for compensating the firstwavefront deviation, characterized in that the compensator includes aphase structure of a material having field angle-dependent properties,the phase structure having the form of annular areas forming anon-periodic pattern of optical paths of different, fieldangle-dependent lengths, the optical paths forming a second wavefrontdeviation compensating the field angle-dependent first wavefrontdeviation, wherein the first wavefront deviation is field curvature. 2.Optical head according to claim 1, wherein said non-periodic phasestructure compensates at least 50% of the root mean square (rms) fieldcurvature wavefront error at a certain field angle with respect to thedirection of an optical axis of the optical system and caused by theobjective.
 3. Optical head according to claim 1, wherein the rmswavefront error caused by the field curvature generated by the objectivelens at a maximum required field angle with respect to the axialdirection, and compensated by the non-periodic phase structure, is lessthan 40 mλ.
 4. Optical head according to claim 1, wherein saidnon-periodic phase structure includes a plurality of annular zones, eachof said zones comprising a step of a substantially constant height withrespect to the shape of the surface of said objective lens on which thephase structure is arranged.
 5. Optical head according to claim 1,wherein the differences between the optical paths are substantiallymultiples of the wavelength of the radiation beam for at least one fieldangle.
 6. Optical head according to claim 4, wherein the radial widthsof said zones are selected in dependence on the amount of fieldcurvature to be compensated.
 7. Optical head according to claim 6,wherein one of said zones (a) has a nonzero height h_(a), measured inrelation to said shape of the surface, located in the region in whichthe normalised pupil coordinate ρ ranges from 0.2 to 0.7.
 8. Opticalhead according to claim 7, wherein a maximum radius of said zone has anormalized pupil coordinate ρ smaller than 0.7,
 9. Optical headaccording to claim 6, wherein one of said zones (b) has a nonzero heighth_(b), measured in relation to said shape of the surface, located in theregion in which the normalized pupil coordinate ρ ranges from 0.7 to1.0.
 10. Optical head according to claim 9, wherein the ratio of saidheight h_(b) of zone (b) and height h_(a) of zone (a) is greater thanone.
 11. Optical head according to claim 4, wherein the heights of saidzones are selected substantially optimally in relation to the fieldcurvature aberration to be compensated for.
 12. Optical head accordingto claim 4, wherein the number of said zones is greater than four. 13.Optical head according to claim 4, wherein the number of said zones isless than ten.
 14. A device for scanning an optical record carrierhaving an information layer, the device including an optical headaccording to claim
 1. 15. Device according to claim 14, including aninformation processing unit for error correction.
 16. An optical devicecomprising an optical system and a compensator, the optical systemimparting a field angle-dependent first wavefront deviation to aradiation beam passing through the system, the compensator beingarranged in the path of the radiation beam for compensating the firstwavefront deviation, characterized in that the compensator includes aphase structure of a material having field angle-dependent properties,the phase structure having the form of annular areas forming anon-periodic pattern of optical paths of different, fieldangle-dependent lengths, the optical paths forming a second wavefrontdeviation compensating the field angle-dependent first wavefrontdeviation, wherein the first wavefront deviation is field curvature. 17.Set of optical elements according to claim 16, wherein the differencesbetween the optical paths are multiples of the wavelength of theradiation beam for at least one field angle.
 18. Set of optical elementsaccording to claim 16, wherein the optical element is a lens.
 19. Set ofoptical elements according to claim 16, wherein the optical element andthe compensator are integrated in a single element.